Guppies and mollies are closely related and very easy to breed. A mature female will give live birth about once a month or so in an aquarium. However, if you want the fry to survive until adulthood, you have to take several steps to care for them, including protecting them from other fish in the tank that may eat them.
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Both guppies and mollies are members of a family of fish called live-bearers. However, mollies have a few special requirements, unlike the hardier guppy. Mollies prefer (and breed better) in water that has some salt in it. They also prefer a temperature of at least 80 degrees Fahrenheit (somewhat warmer than that most fish need, even tropical fish). Under these conditions, a female molly will spawn regularly. A larger tank of around 20 gallons is recommended, particularly for the sailfin molly.
Pick the Right Fish
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The first step to breeding guppies and mollies is to pick the right fish. If you want a specific color, strain or type, consult a breeder. Ordering from a breeder will usually get you better, healthier fish than ordering from a pet shop. Look for large, active fish. You should also feed the parents lots of high-quality fish foods like brine shrimp, bloodworm, fruit flies and other meaty food to get them into spawning condition.
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If you are serious about spawning guppies and mollies, you will need to protect them from their parents. You can do this by either heavily planting the aquarium, giving them lots of places to hide, capturing the babies, or moving the mother to a separate aquarium to spawn. However, moving the babies or mother does stress them out to the point of risking harm to the baby fish. Any tank containing babies should have a sponge filter or other filter that will not suck them into the intake.
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If you want to create new varieties of guppies and mollies, get involved in selective breeding. In selective breeding, you set out to select for certain desirable traits. This entails line-breeding, breeding female guppies with a desired trait back to their fathers. This can "fix" a desired trait, making it come out consistently in your fish. This will usually take multiple tanks and time.